1. Field of the Invention
This invention relates to electroluminescent devices, especially ones that use an organic material for light emission and have thin-film transistors for switch circuitry.
2. Description of Related Technology
One type of electroluminescent device is described in PCT/WO90/13148, the contents of which are incorporated herein by reference. The basic structure of this device is a light-emitting polymer film (for instance a film of a poly(p-phenylenevinylene)xe2x80x94xe2x80x9cPPVxe2x80x9d) sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. The electrons and holes excite the polymer film, emitting photons. These devices have potential as flat panel displays.
Another type of organic light-emitting device is a small molecule device, details of which are given in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference. These have a light-emitting layer which comprises at least one small molecule material such as tris(8-hydroxyquinoline)aluminium (xe2x80x9cAlq3xe2x80x9d) sandwiched between the two electrodes.
In an organic light-emitting device the organic light-emitting layer is generally divided into individual pixels, which can be switched between emitting and non-emifting states by altering-the current flow through them. The pixels are generally arranged in orthogonal rows and columns. Two alternative arrangements for controlling the pixels are generally used: passive matrix and active matrix. In a passive matrix device one of the electrodes is patterned in rows and the other in columns. Each pixel can be caused to emit light by applying an appropriate voltage between the row and column electrodes at whose intersection it lies. In an active matrix display circuitry is provided so that each pixel can be left in an emitting state whilst another pixel is addressed.
FIG. 1 illustrates a circuit for driving one pixel in a thin-film transistor (xe2x80x9cTFTxe2x80x9d) active matrix display. The circuit comprises the pixel itself, illustrated as diode 1, which is connected between electrodes 2 and 3. Electrodes 2 and 3 are connected to all the pixels of the device and a voltage sufficient for emission from the pixel is applied constantly between the electrodes 2 and 3. At least part of the switch circuit 4, which in practice is embodied by thin-film transistors, lies between electrode 3 and the pixel 1. The switch circuit is controlled by way of row and column electrodes 5, 6. To cause the pixel 1 to emit light, voltages are applied to the electrode 6, to switch the switching transistor 7 on, and to electrode 5 to charge the storage capacitor 8. Electrode 6 is then turned off. Since the capacitor 8 is charged the current transistor 9 is switched on and the voltage applied at electrode 3 is applied to the pixel, causing it to emit. Although it requires a more complex circuit than a passive matrix device this arrangement has the advantage that the pixel can be held in an emitting state by means of the capacitor 8 whilst other pixels on different rows and columns are addressed by their row and column electrodes. Using the whole backplane area for thin film transistors and metal lines is well known in transparent and reflective LCD displays.
To allow clearer images to be displayed it is important to be able to control the brightness of each pixel individually, so as to provide a grey-scale. In the active matrix device of FIG. 1 this is done by selecting the voltage applied to electrode 5 and the duration of the pulse applied to electrode 6 so as to fix the charge applied to capacitor 8. The charge on capacitor 8 determines the state of transistor 9 and therefore the current flow to the pixel from electrode 3. The current flow through the pixel determines the brightness of its emission. FIG. 2 shows a graph of current through transistor 9 (I) against gate voltage of transistor 9 (V). There is a flat xe2x80x9coffxe2x80x9d zone 100 where the current and voltage are low and no light is emitted by pixel 1, a sloping transition zone 110 providing intermediate levels of brightness from the pixel 1 and a flat xe2x80x9conxe2x80x9d zone 120 where the transistor is in its fully on state and the pixel is fully on. By fixing the charge on capacitor 8 so that the transistor 9 is at the desired point in the transition zone a desired intermediate level of brightness for the pixel can be achieved.
The shape of the curve in FIG. 2 is determined by the characteristics of the circuit elements, especially current transistor 9. A switch circuit 4 must be provided for every pixel of the display. To achieve the required miniaturisation and low cost the circuit is integrated with the display. However, this arrangement generally leads to great variability of performance between the current transistors of each pixel of the display. Although the current flow in the off and on zones is fairly consistent between current transistors (because the off-current is almost zero and the on-current is largely determined by the resistance of the diode 1), the current transistors"" threshold voltages can differ greatly. This is a particular problem where the light-emitfing material of the display is an organic light-emitting material because the amount of light emitted from organic light-emitting pixels is sensitively dependent on the current through the device. Therefore, for the same input line current different organic light-emitting pixels can produce widely differing intermediate brightnesses. This limits the use of this drive scheme for grey-scale organic light-emitting display devices.
According to a first aspect of the invention there is provided an organic light-emitting having: an organic light-emitting region comprising a plurality of organic light-emitting pixels; first switch means each associated with a respective pixel for switching power to that pixel; and drive means for driving each switch means between a first, low power mode and a second, high power mode, at a frequency sufficient to cause light emission from the associated pixel to appear substantially continuous, the duration of the high power mode relative to the low power mode being variable so as to vary the average brightness of the pixel over the duration of a cycle, wherein in each cycle the high power mode is provided as more than one discrete high power pulse.
According to a second aspect of the invention, the organic light-emitting device may comprise: an organic light-emitting region comprising a plurality of organic light-emitting pixels, each pixel comprising at least two independent light-emitting areas, wherein the light-emitting areas are of different areas; a switch arrangement associated with each pixel and comprising switch means each associated with a respective light-emitting area of that pixel for switching power to that light-emitting area; and a control means for addressing each pixel by its associated switch arrangement and controlling the brightness of each pixel by selectively driving one or more of the corresponding switch means to cause selected ones of the light-emitting areas of that pixel to emit light.